PS234 DPD Reagent #2 Conundrum - Ben?

[elsie]

I received my PS234 Saturday. Yesterday I checked my chlorine with last year's DPD powder and reagent # 871, then did same with the new DPD powder and reagent #2 to ascertain if I could continue to use last year's supplies up before using the new kit (i.e., if the two test results matched I would continue to use up last year's).

On Saturday I notched my chlorine up because I recoated the pool deck with an acrylic stain and invariably a bit of the product splashes off the roller and goes into the water. When I tested yesterday I expected a reading of around 6 ppm.

Indeed, using last year's DPD powder and reagent revealed a ppm of 6 (12 drops to clear the pink). Then I used the new powder and reagent from the PS234. It took 19 drops which translated to a chlorine reading of 9.5 ppm. I then used last year's powder with the new PS234 reagent #2 and again got a reading of 9.5 ppm. I then used the new DPD powder but used last year's reagent #871, and came up with a reading of 6 ppm.

In an effort to figure out why the discrepancy, I used the OTO test. I don't usually use this for a chlorine reading because the DPD test is far more accurate. I filled up half of the vial with distilled water and half with pool water and multiplied the results by 2. I got somewhere between 6.0 and 7.0. So, at a minimum, the new DPD chlorine test is running 2.5 ppm higher than what I think it should.

Lastly, I took last year's reagent #871 and the new reagent #2, held them upside down over the sink side by side, and wasted a few drops of each. It appeared to me that the droplets that come out of #871 are LARGER than the droplets that come out of #2. Indeed, this would account for the difference in readings.

Ostensibly, this could mean the difference between water having 0 CL and having 3.0 ppm.; my brain can't quite wrap itself around the implications for testing for CC.

If last year's DPD test about matches the OTO test, I'm inclined to believe that my "droplet" theory is accurate. Can anyone help out with this conundrum?

[dhale1]

See the following thread

http://www.poolforum.com/pf2/showthread.php?t=1519

[elsie]

Oh bless you, thank you, I feel my sanity inching back. Another idea might be to simply swap out tips with another reagent rarely/never used in the interim once cleaned thoroughly.

Thanks again --

[Watermom]

The only thing about switching tips with some rarely used reagent is that the drop size is supposed to be different for some reagents, so if you switch tips, you won't necessarily be getting the correct drop size to run your test accurately.

[elsie]

Thanks for the advice. I will see if cleaning the tip works until kit owners are sent new tips.